In a fuel cell system, a fuel gas typified by hydrogen gas and an oxidation gas typified by air are supplied to a fuel cell. The fuel cell generates power by a power-generating reaction between the fuel gas and the oxidation gas, and generates water on an air electrode-side. With a proton-exchange membrane fuel cell, an electrolyte membrane is responsible for the conduction of protons during the reaction between the fuel gas and the oxidation gas. Therefore, in order to efficiently maintain the power-generating reaction, a moisture condition of the electrolyte membrane must be monitored and controlled at an appropriate state.
The proton-exchange membrane fuel cell is operated at approximately 60 to 80 degrees C. However, there may be cases where the fuel cell is used in sub-freezing environments. When the fuel cell is left in a shutdown state in a sub-freezing environment, the water content (generated water or water vapor) inside the fuel cell could freeze until a next system activation and, as a result, the next system activation could end up not being preferably performed or could take a prolonged period of time.
Japanese Patent Laid-Open No. 2004-111196 discloses a method for suppressing freezing of a fuel cell after shutdown. With this method, when shutdown conditions are satisfied, the supply of a fuel gas and an oxidation gas is stopped and an output current to be retrieved from the fuel cell is stopped. After the stoppage, dried fuel gas and dried oxidation gas (hereinafter referred to as “dried gases”) are supplied to the fuel cell and a relatively small output current is retrieved from the fuel cell. Subsequently, the supply of the dried gases is stopped and the operation of the fuel cell is stopped.